Muwatta Imam Malik Book of Fasting, Hadith 56

Yahya related to me from Malik from Abu'n Nadr, the mawla of Umar ibn Ubaydullah, from Abu Salama ibn Abd ar-Rahman that A'isha, the wife of the Prophet, may Allah bless him and grant him peace, said, "The Messenger of Allah ﷺ used to fast for so long that we thought he would never stop fasting, and he would go without fasting for so long that we thought he would never fast again. I never saw the Messenger of Allah ﷺ fast for a complete month except for Ramadan, and I never saw him do more fasting in any one month than he did in Shaban.'

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ أَبِي النَّضْرِ، مَوْلَى عُمَرَ بْنِ عُبَيْدِ اللَّهِ عَنْ أَبِي سَلَمَةَ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ عَائِشَةَ، زَوْجِ النَّبِيِّ ﷺ أَنَّهَا قَالَتْ كَانَ رَسُولُ اللَّهِ ﷺ يَصُومُ حَتَّى نَقُولَ لاَ يُفْطِرُ وَيُفْطِرُ حَتَّى نَقُولَ لاَ يَصُومُ وَمَا رَأَيْتُ رَسُولَ اللَّهِ ﷺ اسْتَكْمَلَ صِيَامَ شَهْرٍ قَطُّ إِلاَّ رَمَضَانَ وَمَا رَأَيْتُهُ فِي شَهْرٍ أَكْثَرَ صِيَامًا مِنْهُ فِي شَعْبَانَ .